REAL-TIME GAS-EXCHANGE MEASUREMENT OF OXYGEN-CONSUMPTION IN NEONATES AND INFANTS AFTER CARDIAC-SURGERY

Objectives: The purposes of this study were: a) to measure oxygen cons umption (Vo2.) in ventilated neonates and infants after cardiac surger y, utilizing a real-time gas exchange system; and b) to assess this ne w method by comparing the measured Vo2. with calculated Vo2. (using th e Fick equation and simultaneously determined thermodilution cardiac o utput, measured hemoglobin, and measured mixed venous and arterial sat urations). Design: Prospective, comparison study. Comparison of measur ed Vo2 and calculated Vo2 using correlation coefficient, linear regres sion analysis, and bias and precision. Setting: Cardiac intensive care unit in a children's hospital. Patients: A total of 60 direct compari sons were made between measured and calculated Vo2 in 15 patients (age s ranging from 4 days to 14.1 months with median age of 2.4 months) wh o were receiving mechanical ventilation after undergoing corrective ca rdiac surgery. Interventions: a) Direct measurement of Vo2 using gas e xchange method involving a pneumotachograph and a gas sampling system; b) determination of cardiac output by the thermodilution technique; c ) measurement of arterial and mixed venous oxygen content by blood sam pling. Measurements and Results: The absolute measured Vo2 ranged from 19 to 154 mL/min with a mean of 52 +/- 32 mL/min (when indexed, the r ange was 81 to 367 mL/min/m2 with mean 185 +/- 69 mL/min/m2, or range 4.7 to 18.8 mL/min/kg with mean 10.4 +/- 3.3 mL/min/kg). While 34 (57% ) of 60 measured Vo2 values were within 10% of their respective calcul ated Vo2 values, 58 (97%) of 60 were within 25%; the mean percent diff erence between measured and calculated Vo2 values was 10.6 +/- 7.7%. I n comparing the measured Vo2 and calculated Vo2, the correlation coeff icient was good (r2 = .87; p < .01) and the linear regression equation was: measured Vo2 = 1.1 x calculated Vo2 - 9.0 mL/min/m2. The mean di fference, or bias, was 0 mL/min/m2 with precision of 26 and 52 mL/min/ m2 (at 1 and 2 SD). As an alternative means of examining the measured Vo2 data, we also directly compared the thermodilution cardiac output with cardiac output derived by using the measured Vo2 and the Fick equ ation. The range of Fick-derived cardiac output was between 1.69 to 8. 11 L/min/m2 (mean 3.72 +/- 1.56) and the range of thermodilution cardi ac output was between 1.75 to 742 L/min/m2 (mean 3.71 +/- 1.36). The c orrelation coefficient between thermodilution cardiac output and Fick- derived cardiac output was good with r=.88 (p < .01) and the linear re gression equation was: thermodilution cardiac output = 0.81 x Fick-der ived cardiac output + 0.71 L/min/m2. The bias was -0.01 L/min/m2 with a precision of 0.54 L/min/m2 at 1 SD (or 1.08 L/min/ml for 2 SD). Conc lusions: Measured Vo2 using a gas-exchange system compared favorably w ith calculated Vo2 values using the Fick equation and simultaneously o btained thermodilution cardiac output and arterial and venous oxygen c oncentrations. By employing this breath-by-breath gas-exchange system, real-time Vo2 measurement in ventilated neonates and infants is now f easible.